Respiratory and anesthetic gas monitoring has achieved a high standard of technological advancement with the development of monitoring techniques that enable quick diagnosis and treatment of unfavorable trends in the condition of a patient, improved survival rates, early extubation following surgery and shorter times in intensive care units. Applications of respiratory gas and anesthetic agent monitoring include the measurement of oxygen consumption, carbon dioxide production, anesthetic agent uptake and the detection of anesthesia machine circuit disconnections and introduction of air emboli into the blood. Continuous analysis of patients' respiratory and anesthetic gases is becoming increasingly important in improving patient safety during the course of treatment. For example, breath-by-breath monitoring of the concentrations and identity of the anesthetic agents present in a patient's respiratory gases leads to a more scientific basis for the administration and control of the anesthetic agents.
Continuous, breath-by-breath monitoring of a patient's respiratory gases and simultaneous determination of multiple specific respiratory gases and anesthetic agents in the patient's system can often facilitate diagnosis and treatment, anticipate and prevent the development of oncoming problems, and otherwise provide instant data for physicians and other health care personnel to use in therapeutic situations. Measurements of any gas of interest in a patient's breath can be sampled on a continuous basis and monitored by an appropriate type of gas analyzer. For example, when monitoring a patient's carbon dioxide level, a sharp reduction of carbon dioxide in the breath might indicate an imminent failure of respiration. Similarly a sharp increase in the level of carbon dioxide might be an indication of other conditions requiring attention.
Respiration monitoring of patients is now available utilizing many types of commercially available gas analyzers including infrared (IR), polaragraph, mass spectrometer (MS), Raman spectrometer, etc. Due to the high cost of some of the monitoring equipment, a single monitor may be connected to several patients simultaneously. In many of these situations, the gas analyzer is placed in a remote location and lengthy capillary tubes are used to connect the patients to the analyzer unit. Since it is common practice to humidify inspired gas, and since the expired gas from the patient is often at nearly 100% relative humidity and 37 degrees centigrade, water can easily condense at room temperature in the tubing interfacing the patient with the analyzer. Virtually all commercially available gas analyzers (e.g., IR, polaragraph, MS, Raman spectrometer, etc.) are adversely affected if condensed water or other liquids enter the detector portion of the analyzer. Additionally, the presence of water vapor in samples of expired gas can be a source of error when making measurements of expired gas concentrations.
One prior method for removing water vapor from expired gases prior to analysis physically drys the expired gas, for example, by introducing the expired gas into a desiccator. One such system has been developed using a desiccator filled with calcium sulfate (CaSO.sub.4) as the drying agent. Such desiccator systems experience at least two significant problems. First, the drying agent must be carefully monitored and replaced on a regular basis when it is depleted. Second, the large desiccator volume required to perform the drying of the gas makes for increased dead space within the system and thus results in longer "washout times" for measuring changes in gaseous composition. The term dead space refers to any space in the system between the point where the sample is tapped from the patient and the point at which the sample enters the gas analyzer. This would include the space within the connecting tubes, filters, desiccants, valves, traps etc. The term "washout time" refers to the amount of time which is needed for a unit of gaseous sample to wash out or displace the gas already within the system.
Washout time is an important factor in monitoring changes in the concentrations of oxygen, carbon dioxide, anesthetic agents and other constituents of the patient's inspired and expired gases. Where large total volumes or dead volumes are present within a metabolic gas monitoring system, corresponding large washout times are created, resulting in decreased ability to quickly and accurately measure time dependent changes in the composition of the inspired and expired gases. The long washout times associated with desiccator systems do not allow for the dynamic response necessary to measure time dependent changes in the oxygen and carbon dioxide concentrations in breath-by-breath analysis of expired gas. The large total volumes and dead volumes of desiccator systems have resulted in less sensitivity to changes in the composition of the gases analyzed and less accurate measurements of the oxygen, carbon dioxide and anesthetic agent components of the gases. Thus, systems which use drying agents to remove the water vapor often require frequent replacement of the drying agent and often introduce long delay times in the sampling line precluding the acquisition of breath-by-breath data. Additionally, in some applications, the drying agent may absorb the gas being monitored and lead to inaccurate measurements.
Another common technique for removing moisture, patient secretions and other liquids from gas samples employs cold traps in the gas line to condense the moisture. Condensation techniques have generally not been completely successful due in part to the excessively large dead spaces which are added to the gas transport system by the cold traps. The large dead space and long washout time problems previously described with respect to desiccator systems also apply to cold trap systems.
One such cold trap system is described in the article entitled "A DRYER FOR RAPID RESPONSE ON-LINE EXPIRED GAS MEASUREMENTS" by N. S. Deno and E. Kamon. This article discloses a water condensation method for drying an expired gas sample before it reaches an analyzer. The disclosed method utilizes a dryer which consists of an ice bath condenser and a separator for removing the condensed water. As with other cold trap condenser devices, this approach has long response times which reduce the utility of the device for breath-by-breath analysis measurements.
The article entitled "ELIMINATING THE EFFECT OF WATER VAPOR IN RESPIRATORY GAS ANALYSIS" by Larry G. Wong and Dwayne R. Westenskow reports a method for partially eliminating water vapor from expired air samples by cooling the gas samples to a known temperature. This technique substantially minimizes the effects of water vapor pressure on O.sub.2 measurements. Based on Dalton's law of partial pressures, the vapor pressure of water at a given temperature and pressure is constant and is independent of other gas concentrations. The effects of water vapor in expired gas analysis are reduced by bringing all sampled gases to a specific lowered temperature. By reducing the temperature of the saturated gas sample to a known temperature, the water vapor partial pressure may be determined. Thus, when gases are sufficiently equilibrated with temperature in an appropriate apparatus, water vapor pressure is constant. However, the above-described system is not suited for breath-by-breath measurements because large dead spaces degrade several desired performance characteristics including response time. Furthermore, this system, while removing a small quantity of moisture from the sample, is not primarily a water removal system and allows most of the water vapor contained in the gas sample to enter the gas analyzer.
U.S. Pat. No. 4,090,513 entitled "HEAT AND MOISTURE EXCHANGING DEVICE FOR RESPIRATION" discloses a device for removing moisture from a tube carrying respiratory air. The moisture accumulates on an exchange layer and flows out of the device through a drainage tube. As with other condensation moisture removal systems, decreased response time and water removal considerations limit the applications for which this system is suitable.
Another approach for preventing liquids from reaching a monitoring device is disclosed in U.S. Pat. No. 4,485,822 entitled "MULTI-PHASE INTERFACING SYSTEM AND METHOD". This patent discloses a patient interface which relies on a disc-shaped hydrophobic filter to prevent fluids from entering the analyzer. This system overcomes the poor response time of most cold trap and desiccator systems by using a low dead space volume disc filter. In such an interface, however, in the event that the patient requires a humidifier, enough water can condense within the filter to occlude gas flow through the filter. In some cases, this can occur within 30 minutes, depending on the gas flow rate to the analyzer and the ambient temperature. Not only is this inconvenient for the medical staff and patient, since the filter must be changed frequently, but this can also result in a loss of vital medical information during the time period that the filter is being changed.
U.S. Pat. No. 4,549,553 entitled "APPARATUS AND METHOD FOR USE IN A MEDICAL GAS SAMPLING SYSTEM", discloses an approach for providing a sample gas flow from an air tube to a patient undergoing automatic ventilation. The air tube includes a gas diffusive membrane disposed in a wall of the air tube. The gas diffusive membrane may be made of water absorbing or water passing materials to eliminate excess water from accumulating and blocking the gas sample flow. In a preferred embodiment a non-wettable gas diffusive membrane is used to prevent or reduce water from entering the sample gas flow; otherwise, water saturates the membrane and water eventually passes through the membrane to enter the gas sample flow. By non-wettable it is meant that the membrane resists or cannot be saturated with liquid and its surfaces resist or cannot be covered with liquid. A Teflon.RTM. mesh membrane is a suitable non-wettable membrane for this application. Teflon.RTM. is a registered trademark of E. I. Du Pont de Nemours & Company, Inc in Wilmington, Del. This approach, while perhaps reducing the frequency of sample flow blockage, does nothing to remove the moisture from the system and is still susceptible to occlusion in long term usage. Additionally, the embodiment shown in FIG. 1A incorporates a large volume funnel 20 which increases the dead space volume in the gas transport system and results in poor response time characteristics.
A more recent approach for preventing water from condensing in the gas transport circuitry uses interface tubing comprising a polymer that is highly permeable to water vapor, but simultaneously has a very low permeability for the respiratory and anesthetic gases being analyzed. One such polymer was developed by Du Pont scientists and is marketed in tubing form as Nafion.RTM. tubing. Nafion.RTM. is a registered trademark of E. I. Du Pont de Nemours & Company, Inc. in Wilmington, Del. By attaching the Nafion.RTM. tubing directly to the patient breathing circuit near the connection to the patient, a majority of the water vapor in the sample gases diffuses out before it arrives at the filter barrier. The disadvantage of this approach is the high cost of the Nafion.RTM. tubing. A section of Nafion.RTM. tubing of sufficient length to remove the desired amount of water vapor is very expensive in relation to the cost of the other components comprising the patient interface. Therefore, it is generally considered a reusable rather than a disposable gas sampling line and, as such, requires cleaning and sterilization between uses.
Many of the attempts heretofore to remove liquids from patient interfaces introduce levels of dead space into the interfacing system which adversely affect the accuracy of the results produced by the monitoring system. Other attempts which overcome the dead space problems either 1) utilize expensive materials which, when incorporated into a disposable patient interface, make the interface very expensive for a one time use item, or 2) are subject to blockage in short periods of time.
The proliferation of respiratory gas monitoring techniques and the increased demand for breath-by-breath respiratory gas monitoring accentuate the need for a patient interfacing system which prevents moisture from reaching the detection portion of the gas analysis systems. Such a patient interface system should reduce or eliminate the frequency of occlusion or contamination of the gas transport system and detection portion of the gas analyzer. This system should eliminate or reduce the risk of water vapor condensing within and clogging components of the system such as the inline filter or the analyzer gas cell. In addition, the patient interface system will desirably provide a barrier to prevent contamination in the form of secretions, condensed water, and particulates from entering the gas analyzer. It is preferred that such an interfacing device be a one time use item for hygienic reasons. However, for economic reasons, a one time use item must also not be prohibitively expensive if is to be disposed of after a single use. The device of the present invention satisfies all of these requirements.